1. Field of the Invention
The present invention relates to a communication technology, and it particularly relates to a communication method for performing communications between two radio apparatuses according to channel characteristics and a radio apparatus utilizing said method.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize high-speed data transmission and are robust in multipath environments. This OFDM modulation scheme has been used in the wireless standards such as IEEE802.11a/g and HIPERLAN/2. Packet signals in such a wireless LAN are generally transmitted through a time-varying channel environment and are also subject to the effect of frequency selective fading. Hence, a receiving apparatus carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a packet signal. One is the known signal, provided for all carries in the beginning of the packet signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in a data area of the packet signal, which is the so-called pilot signal.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is a MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and a plurality of packet signals to be transmitted in parallel are set (hereinafter, each of data and the like to be transmitted in parallel in the packet signal is called “stream”). That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates. Moreover, combining this MIMO system with the OFDM modulation scheme results in a higher data transmission rate.
In the MIMO system, the data rate can also be adjusted by increasing or decreasing the number of antennas to be used for data communications. Furthermore, the data rate can be adjusted in greater detail by applying adaptive modulation to the MIMO system. Communications according to channel characteristics between a transmitting apparatus and a receiving apparatus are performed in adaptive modulation. Thus communications suited for the channel characteristics are performed. To enhance the processing accuracy of adaptive modulation, it is desired that channel characteristics between a plurality of antennas included in the receiving apparatus and a plurality of antennas included in the transmitting apparatus be acquired. To enhance the accuracy in acquiring the channel characteristics, a transmitting apparatus or receiving apparatus sends known signals for use in channel estimation, from all of the antennas. Hereinafter, known signals, for use in channel estimation, assigned to a plurality of streams are called “training signals” independently of the number of streams to which data are assigned. For example, even in the case when data are assigned to two streams, training signals are assigned to four streams.
Under these circumstances, the inventor of the present invention came to recognize the following problems to be solved. In general, the adaptive modulation is executed based on EVM (Error Vector Magnitude), receiving strength and the like. However, when interleaving or error correction is being applied to a communication system such as a MIMO system, there are cases where the modulation scheme or coding rate of error correction suited to the then channel characteristic differs even if the same EVM value or receiving strength value is exhibited. More specifically, the effect of error correction differs in response to the magnitude of delay characteristics in a radio channel.